Mine sprung out as well.. There is a hole that the spring+plastic piece sits in, put it in there and move the rotary knob over it such that it stays in place when reassembling..

Thanks! So for me, I noticed that the motor rotates but the blade would not move sometimes. I would shake it, and then it would "catch on" briefly... I hit the shaft of the motor with some sandpaper and superglue, looks to be working, hope it holds. (I could just pull the metal piece attached to the shaft off, not sure how it is supposed to attach originally - it is a simple round shaft without a key or anything.